RFR 8135248: Add utility methods to check indexes and ranges

Roger Riggs Roger.Riggs at Oracle.com
Mon Sep 21 14:17:14 UTC 2015

Hi Paul,

java.util.Arrays.java: line 5236:  new IndexOutOfBoundsException()
  - It is always appreciated when debugging to be given a message with 
the index that is out of range and the range expected.

When converting existing code, is it expected that the exception 
messages will be unchanged?

I can't quite visualize how this would be applied to existing code.
Will there be any benefit to adding static checking methods to the 
typically thrown exceptions
that can be used as method references instead of the implicitly defined 


  - line 78, 91, etc  "withing" -> "within"


On 9/21/2015 9:42 AM, Paul Sandoz wrote:
> Hi,
> Please review the following which adds methods to Arrays to check indexes and ranges:
>    https://bugs.openjdk.java.net/browse/JDK-8135248
>    http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8135248-array-check-index-range/webrev/
> The original motivation was an intrinsic method, Arrays.checkIndex, to check if an index is within bounds. Such an intrinsic guides HotSpot towards better optimisations for bounds checks using one unsigned comparison instead of two signed comparisons, and better eliding of integer to long conversions when an index is used to create an offset for Unsafe access. The end result is more efficient array access especially so from within unrolled loops. The VarHandles work will use Arrays.checkIndex for array access.
> A follow up issue [1] will track the intrinsification of Arrays.checkIndex.
> We thought it would be opportunistic to support two further common use-cases for sub-range checks, Arrays.checkFromToIndex and Arrays.
> checkFromIndexSize. There is no current plan to intrinsify these methods.
> Bounds checking is not difficult but it can be easy to make trivial mistakes. Thus it is advantageous to consolidate such checks not just from an optimization perspective but from a correctness and security/integrity perspective.
> There are many areas in the JDK where such checks are performed. A follow up issue [2] will track updates to use the new methods.
> The main challenge for these new methods is to design in such a way that
> 1) existing use-cases can still report the same set of exceptions with the same messages;
> 2) method byte code size is not unduly increased, thus perturbing inlining; and
> 3) there is a reasonable path for any future support of long indexes.
> Paul.
> [1] https://bugs.openjdk.java.net/browse/JDK-8042997
> [2] https://bugs.openjdk.java.net/browse/JDK-8135250

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